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Radiation Measurements

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Health Physics
2: Radiation Measurements
Introduction
• Radiation not detected with our senses
• Need detectors to confirm presence
of radiation
• Avoid over – exposures (reddening of skin
- 3Gy)
Page(s): 107
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Detection of Radiation
• Made possible by its interaction with matter
(solid, liquid gas)
• Ionization (electrical charges), excitation
• Direct (charged particels) and indirect
(photons, neutrons) ionization
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Indirect Ionization by Photon
Ejected
Electron
Incoming
Photon
Two Basic Types of Radiation
Measurements in Health Physics:
• External radiation hazard
measure exposure rate, dose or dose-rate
• Internal radiation hazard
measure contamination in working area, bioassay
Page(s): 107 to 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Penetration Power of Radiation
External Radiation Hazard (1)
• Discriminate between particles and gamma
radiation using probe - shield
• Measure exposure rate (X/t) or dose rate
(mR per hour, mSv per hour)
• Measure dose (integrate dose rate,
dosimeter)
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
External Radiation Hazard (2)
continued …
• X-rays, gamma radiation, neutrons
• Energetic beta particles (P-32: 1.7 MeV)
• Neutrons (from accelerators, cyclotrons),
fast and thermal neutrons
Page(s): 107 to 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Internal Radiation Hazard (1)
• Measure contamination in working area
(surface, air, water) “wipe tests” (betas)
• Whole-body counter (gamma emitters)
• Bioassays (thyroid assay, urine analysis)
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Internal Radiation Hazard (2)
continued …
• Alpha or beta particles when inhaled or ingested
(e.g. tritium vapors in power stations containing
H-3 with 18keV betas)
• Boneseekers with long half-lives when inhaled or
ingested
(Sr-90: 0.5MeV betas, Pu-239 : 5MeV alphas)
• Any radioactive material that enters the body in large
amounts
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Types of Radiation Monitoring
• Area and survey monitoring (portable or
fixed detectors)
• Technique or procedure monitoring
(DRDs or EPDs)
• Personal Monitoring (TLD “badges”)
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
“Ideal” Radiation Detector
• Responds to one radiation type only
• Includes radiation quality factor, wR
• Uniform energy response
• Gives equivalent dose (H) or equivalent dose
rate
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
“Real” Radiation Detector
• Need to discriminate between particles and
gamma radiation using probe - shield
• Non-uniform energy response
• Often gives exposure rate (X / t) only
(Milli-Roentgen per hour)
Page(s): 108
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Dependence of Gamma
Survey Meter
Page(s): 153 to 154
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
f-Factor (rads/Roentgen)
Page(s):
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Radiation Instruments
GMs from 1962 to 1999
1985
1999
1970
1962
Instruments
Example: GM Model
GM Survey Meter
• Dial in mR/hr
• Battery check
Electronic Personal Dosimeter(EPD)
Page(s): at end of handout
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Electronic Personal Dosimeter(EPD)
Skin dose
Body dose
Radiation Instruments
Car Gate
Radiation Instruments
Conveyor
Radiation Instruments
Truck Monitor
Radiation Instruments
Security Gates
Gas Detectors
• Ionization Chambers
• Proportional Counters
• Geiger-Mueller Counters (GMs)
Page(s): 111 to 125
Page numbers refer to handout:”Chapter 8: Radiation
Measurements”.
Gas-Filled Detectors
Voltage Source
Incident
Ionizing
Radiation
+
-
+
-
+
-
Anode
+
Cathode -
Electrical
Current
Measuring
Device
Ionization Chamber
Page(s): 113
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Ionization Chamber
• Characteristics
– rel. low sensitivity (ideal as control
instrument in high field of nuclear reactors)
– measures exposure rates up to 1000 R / min
•
Page(s): 112 to 117
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Condenser Type Dosimeter
Page(s): 115
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Direct Reading Dosimeter (DRD)
Natural
leakage of
5-10 mR/day
Keep control
DRD in desk!
Do not drop!
Page(s): 115-116
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Gas Multiplication
secondary ions
Page(s): 117 to 118
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Proportional Counters
windowless
Page(s): 118 to 119
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Proportional Counter
• Characteristics
– Energy information preserved
– Particles yield larger pulses than photons
– Differentiate particle exposure in presence
of photons
– Detects thermal neutrons via n-alpha
reaction if tube lined with Boron or if BF3 is
used as filling gas
•
Page(s): 117 to 119
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Geiger Plateau
Page(s): 120
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Geiger-Mueller Counter
Page(s): 119 to 124
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
GM Counter
• Characteristics
– large dead time (~ 100μs), saturation
– has no energy info.
– high sensitivity (100% for each ionizing event)
– measures low exposure rates (~0.1 mR / hr)
•
Page(s): 112 to 117
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single Images
Gas Detectors:
Summary
• Ionization chamber has relatively low
sensitivity, good for high radiation fields, has
energy info.
• Proportional counter as neutron detector
with BF3 as filling gas (slow neutrons
undergo n-alpha reaction). Has energy info.
• GM has large dead time (~100 micro-sec),
saturation in high radiation field, very
sensitive, no energy info.
•
Page(s): 111 to 125
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Scintillation Detectors
• Phosphors (NaI(Tl), CsF, BGO, LSO)
• Photomultiplier Tube (PMT)
dynodes, counting chain, spectra
• Liquid Scintillation Counting (“wipes”)
Page(s): 125 to 137
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Photon Interaction with NaI(Tl) Crystal
Page(s): 126 to 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
NaI(Tl) – PMT Assembly
Page(s): 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Scintillator Characteristics
•
•
•
•
•
•
Phosphors (NaI(Tl), CsF, BGO, LSO)
Photoelectric interaction ~ Z4
NaI(Tl): reference, decay const. ~ 1Ојs
CsF : faster than NaI(Tl), TOF PET
BGO : slower but more efficient, PET
LSO : very fast (~1ns), high res. PET
Page(s): 125 to 137
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Phosphor- PMT Assembly
Page(s): 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Photomultiplier Tube (PMT)
Page(s): 127 to 129
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Electron Multiplication in PMT
Page(s): 127 to 129
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Counting Chain (1)
Page(s): 129
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Discriminator Action
Page(s): 130
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Counting Chain (2)
Page(s): 131
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Counting Chain (3)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Co-60 Energy Spectrum from NaI(Tl)
Detector
Page(s): 136
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Spectrum from NaI(Tl) Detector
Page(s): 136
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Resolution (FWHM)
Page(s): 136
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Transfer in Phosphor
Page(s): 125 to 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Photoelectric Effect
Page(s): 125 to 127
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Well Counter
Page(s):
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation Counter (1)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation Counter (2)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation Counter (3)
Page(s): 132
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Liquid Scintillation
• Scintillator in intimate contact with radiation
source (mainly alphas and betas)
• Solvent (toluene) and solute(POPOP)
• Efficiency for alphas and betas: 50 to 100%
• Correct for quenching effects (chemical,
color)
• Wave length shifter to match photocathode
response
• Page(s): 132 to 133
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Scintillators for Alpha and Beta
Particles
Page(s): 134 to 135
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Films and TLDs
• Film dosimeters (badges)
body, skin, wrist monitoring
• Thermoluminescent Dosimeters (TLDs)
LiF, Al2O3 in many shapes: finger ring TLD
very sensitive, linear response
neutron response possible (Li-6, Li-7)
Page(s): 138 to 147
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Film Dosimeter Calibration Curve
Page(s): 138 to 139
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Film Dosimeter: Energy Dependence
Page(s): 140 to 141
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Film
Dosimeters
Single
Images
• Gamma exposure from ~ 10mR to 1000R.
• Need filters to correct energy dependence and
yield beta exposure.
• Cd filter allows thermal neutron dose meas.
• Wearing period of a few months.
• Pros: cheap, permanent record, easy
processing.
• Cons: darkening by humidity, heat, H-3 vapor.
•
Page(s): 138 to 142
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
TLD: X-ray Sensitivity of LiF
Page(s): 143 to 147
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
TLD
Dosimeters
Single
Images
• Gamma and beta exposure from ~ 20mR to
106 R
• No energy dependence
• Linear dose response over wide range
• Tissue equivalence
• Thermal neutron dose meas. possible
– (Li-6, Li-7)
• Wearing period of up to one year.
• Cons: no permanent record, no visual record
•
Page(s): 143 to 147
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Special Detectors
• Semiconductor detectors (nuclear diodes)
Si(Li), Ge(Li), Ge(hyperpure)
• Thermoluminescent neutron dosimeters
Li-6 vs. Li-7
• Damage track neutron dosimeters
• Bubble neutron dosimeters
Page(s): 147 to end of handout
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Semiconductor Detector
Si or Ge
p-layer
n-layer
Page(s): 147 to 149
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Detector Cooling
Page(s): 147 to 149
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Semiconductor
Detector
Single Images
•
•
•
•
Acts like a solid state ionization chamber.
Si(Li) (delta E=1.2eV), room temp.
Ge(Li) (delta E= 0.7eV), cool to 77o K .
Hyperpure Ge, cool to 77o K, recyclable.
• Wion-pair ~ 3.5eV : high sensitivity and
energy resolution (~1%).
• Tissue beta and gamma dose rate (EPD).
•
Page(s): 147 to 148
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Neutron TLD with Li-6 and Li-7 (2)
• Li-6 (7.5%): responds to both gammas and to
slow neutrons by n- alpha reaction enrich!
• Li-7 (92.5%): only responds to gammas
• Polyethylene slows down fast neutrons
• Cd captures slow
neutrons
•
Page(s): 149 to 150
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Neutron TLD with Li-6 and Li-7 (1)
Page(s): 149 to 150
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Damage Track Neutron Dosimeter
Page(s): 149 to 151
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Bubble Neutron Dosimeter
• Elastic polymer with suspended
droplets of superheated liquid
• When struck by radiation,
droplets form gas bubble
• Bubbles remain fixed in polymer
for permanent visual record
• Calibration in “bubbles per mrem”
or “bubbles per Sv”
Page(s): end of handout
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Use of Radiation Instruments
• Detection and geometric efficiency
• Time constant and dead time
• Directional response
• Operational checks (battery!), calibration
Page(s): 151 to 156
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Single
Proper Use
of Images
Rad. Instr.(1)
• Remember: counter efficiency depends on:
– geometric efficiency (1/r2)
– detection efficiency (particles vs. photons)
– detector entry window
– detector dead time (~100μs for GM, <1μs
for NaI)
• Respect �time constant’ of instrument (wait ~3
time constants for accurate reading). Use
�fast / slow’ switch.
•
Page(s): 151 to 15
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Energy Dependence of Gamma
Survey Meter
Page(s): 153 to 154
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Rate Meter Response (Time
Constant)
Page(s): 152 to 153
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Proper Use
of Images
Rad. Instr. (2)
Single
• Respect directional response!
– Example: beta window on dosimeter
• Respect warm-up time:
– transistors need none
– PMTs need a few minutes
•
Page(s): 151 to 155
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Proper Use
of Images
Rad. Instr. (3)
Single
• Routinely perform operational checks:
– battery verification (see mark on dial)
– ratemeter check (pulse generator)
– calibration (built-in check source, official
agent)
Page(s): 151 to 155
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Master
Page(s): 107 to
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
Shielding
BASIC KNOWLEDGE
- DOSE The Dose From Being Exposed to Cosmic
and Machine Produced Radiation Depends
on:
• Time
• Distance
• Shielding
Energy Response of Ionization Chamber
Page(s): 116 to 117
Page numbers refer to handout:”Chapter 8: Radiation Measurements”.
References
• Nuclear Regulatory Commission Home Page:
www.nrc.gov
• teachers
corner@www.nrc.gov/NRC/teachers.html
• students
corner@www.nrc.gov/NRC/STUDENTS/stude
nts.html
•
Nuclear Energy Institute Home Page: www.nei.org
• science club@
www.nei.org/scienceclub/index.html
• Health Physics Society Home Page: www.hps.org
• www.hps.org/publicinformation/radfactsheets/
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